The present invention generally relates to a scanning type image forming apparatus capable of changing a dotting density, and particularly to a scanning type image forming apparatus capable of changing a dotting density which is used as an outputting unit of office automation machines such as personal computers, engineering workstations (EWS), word processing machines, digital copy machines and high-speed facsimile machines.
The scanning type image forming apparatus, such as a laser beam printer, has a scanning unit in which a light beam (laser beam) modulated on the basis of image data scans an image forming medium such as a photosensitive material. For example, in the laser beam printer, the laser beam is modulated on the basis of the image data and in synchronism with a writing clock signal supplied from a controller. One clock (one cycle) of the writing clock signal corresponds to one pixel in the image. The laser beam printer described above can rapidly form an image having a high resolution.
In addition, in the laser beam printer, it is possible to easily change a dot size in the image, each pitch between dots and each pitch between scanning lines. That is, a dotting density can be changed. The dotting density is defined as the number of pixels (dots) per unit length in the image which should be formed. Thus, the laser beam printer having a function for selecting a dotting density from a plurality of dotting densities has been proposed.
Prior art related to the changing of the dotting density has been disclosed in Japanese Laid Open Patent Application Nos. 59-117372, 60-120658, 60-93873 and so on.
In the image forming apparatus disclosed in Japanese Laid Open Patent Application No. 59-117372, the dot size and the writing clock signal are controlled on the basis of the pitch between dots and the number of pixels (dots) in one image unit such as one character so that the dotting density is controlled. In the image forming apparatus disclosed in Japanese Laid Open Patent Application No. 60-120658, the intensity of the laser beam and the scanning speed are respectively controlled on the basis of a magnification of the image which should be formed so that the dotting density is controlled. In the image forming apparatus disclosed in Japanese Laid Open Patent Application No. 60-93873, a plurality of optical systems which have different resolutions (corresponding to the dotting densities) are prepared. When one of the optical systems is mounted in the image forming apparatus, the dot size and the scanning speed are controlled on the basis of the resolution of the optical system which is mounted in the image forming apparatus.
In the conventional scanning type image forming apparatus, the writing clock signal and/or the scanning speed are controlled so that the dotting density is changed. However, since one clock of the writing clock signal corresponds to one pixel (dot), in the image forming apparatus in which the timing for supplying the image data to the scanning unit is controlled on the basis of the writing clock signal, when only the writing clock signal and/or the scanning speed are controlled to change the dotting density, the starting point and the width of the image which is formed in a main scanning direction are changed.